This invention relates generally to circuit breakers, and more particularly, to motor circuit breakers including a magnetic trip unit.
Circuit breakers are used to provide protection against very high currents produced by short circuits. At least some known circuit breakers include trip units which include a rotationary stator-armature system and a stator-flapper gap system. The stator-flapper gap system is variable to control a gap within the trip unit to facilitate overcurrent trip protection for different trip settings. More specifically, because a magnitude of a low current setting is smaller than that of a high current setting, the gap is defined more narrowly for low current protection, and larger for higher current protection.
To accomplish varying the gap, flapper return springs are used in an arrangement such that the restoring moment is increasing with a decreasing gap. However, because the spring force may diminish over time, accurate repeatability of such a flapper return system may diminish. Eventually, continued operation of such a return spring system may cause a disengagement of the latching system, thus increasing a potential of unnecessary motor trips. Furthermore, because of space considerations, the air gap may not be defined wide enough to satisfy the initial desired high current settings, and as such, the circuit breaker may trip at a lower current setting that desired.
In one aspect, a motor protection trip unit for a circuit breaker includes an electrically conductive strap, a yoke including a body including a first side wall, a second side wall, a third side wall, and a fourth side wall. The third side wall extends between the first and second side walls, the fourth side wall extending from the first and second side walls and comprising a yoke gap extending therethrough. A motor protection trip unit for a circuit breaker also includes a flapper holder coupled to the strap, a flapper pivotally coupled to the flapper holder, and a biasing mechanism coupled between the flapper holder and the flapper. The biasing mechanism is configured to maintain the flapper a distance from the yoke such that a yoke-flapper gap is defined between the yoke and the flapper.
In another aspect, a circuit breaker includes a motor protection trip unit including a yoke, a flapper, and a conductive strap. The trip unit including the conductive strap extending therefrom and proximate to the flapper, the flapper and the yoke coupled to the trip unit, the yoke including four side walls defining a yoke gap, the flapper configured to rotate independently of the trip unit to selectively adjust a gap defined between the yoke and flapper. The trip unit also includes a contact arm configured to engage the conductive strap, a latching assembly coupled to the contact arm, a tripping assembly rotatable about an axis of symmetry and configured to engage the latching mechanism for tripping the circuit breaker during an overcurrent condition.
In another aspect, a method of operating a motor is provided. The method includes providing a rated current to the motor through a circuit breaker that includes a tripping assembly, sensing a low current three times rated current and a high current of twenty times rated current with a motor protection trip unit that is coupled to the circuit breaker and the tripping assembly, and tripping the motor protection trip unit when an overcurrent condition is sensed such that current flow to the motor is prevented.